This invention relates to a Hall proximity detector, and especially to a Hall ferrous-gear-tooth sensor with an attached magnet capable of detecting the near presence of each tooth of an adjacent rotating ferrous gear, and more particularly relates to such a Hall sensor capable of detecting the leading and trailing edge of each passing tooth while being insensitive to DC components of the Hall signal within the sensor circuit including that caused by the magnet and those attributable to DC offsets in the Hall element(s) and the Hall-signal amplifiers.
It is known to provide a Hall sensor circuit that detects a passing magnetic or a ferromagnetic article wherein the amplified Hall signal is amplified and then filtered by an AC coupling means such as a high-pass filter to remove the DC component of the Hall signal. Removal of the DC component is especially necessary when a magnet is attached to the Hall sensor to provide a magnetic field that will be distorted when a ferrous article passes by the sensor, which distortion is to be sensed by the Hall sensor.
A number of such AC-coupled Hall sensors are disclosed in the patent U.S. Pat. No. 4,218,659, issued Aug. 19, 1980. Another such AC-coupled Hall sensor employs a differential high-pass filter having but one filter capacitor in the Hall-signal path, and is described in the patent to Ramsden U.S. Pat. No. 4,982,155, issued Jan. 1, 1991 and assigned to the same assignee as is the present invention.
These Hall sensors with high pass filters tend to produce a false triggering and false output when the power is turned on, because the filter capacitor(s) must charge to the appropriate DC level before normal operation can proceed. What DC level is appropriate cannot be sensed, e.g. because at power up it is not known whether the Hall signal level first existing is attributable to peaks, i.e. a tooth or valley, of the gear or how much of the first seen signal level reflects amplifier off-set. There are no known supplementary power-up fast-charge circuits that will solve this problem.
Further, the voltage of the high pass filter capacitor during sensor operation at low frequencies will decay tending to cause the sensor circuit to false trigger. It is much preferred that a sensor fail to produce an output at all, rather than to produce a false output.
It is an object of this invention to provide a Hall sensor that overcomes the above-noted short comings of the prior art.
It is another object of this invention to provide an improved Hall sensor for detecting the leading and falling edges of the Hall voltage signal employing a Hall-signal track-and-hold circuit by sensing the occurrence of a Hall-signal pulse slope or peak wherein the DC component of the Hall voltage is ignored, the off-sets of Hall element(s) and Hall signal amplifiers are ignored, and only one small Hall-signal-modifying capacitor is employed.
It is a further object of this invention to provide such a sensor wherein the capacitance of the small capacitor is many times smaller than needed in the prior art capacitor(s) in the Hall-signal-modifying path of the track-and-hold circuit, for a given low frequency gear-tooth-counting cut-off point, e.g. a given minimum gear-tooth passing rate.